We have continued our research on membrane fusion, the fundamental step in secretion, viral infection, fertilization and neuro-transmission. Sea urchin cortical granule exocytosis, an example of calcium triggered membrane fusion, has served as a model system. The sea urchin planar isolated cortex is the only model system that exists where the kinetics of exocytosis can be studied in the absence of reserve granule mobilization to docking sites or subsequent endocytotic activity. We have developed a two parameter model which describes both the kinetic and steady-state features observed in sea urchin cortical granule exocytosis. The kinetics of the fusion process are fit by our kinetic model and the maximal rate and extent of fusion reached a plateau at different calcium concentrations, in agreement with the model. We have determined from the calcium dependence of cortical granule exocytosis, and from the exposure time and concentration dependence of NEM inhibition, that 1) sea urchin egg cortical granule-plasma membrane docking sites have on average nine fusion complexes, 2) fusion complexes are randomly distributed among the population of exocytotic granules, and 3) calcium-secretion coupling is an intracellular manifestation of 'receptor reserve'. The relationship between docking/activation and subsequent calcium triggered fusion was examined by altering the pH of solutions matched for free calcium. Even though granules easily detach from the plasma membrane in the absence of calcium at pH greater then 7.5, no changes in the calcium activation curves were detected at pH 7.4.